CAD Welding | CADWELL Thermite Welding | Exothermic Welding

Cad welding / Cadwell thermit welding process:

Cad welding or Cadwell Welding process is a modification of the simple thermit welding process (exothermic welding process) in which controlled reduction of copper oxide and aluminium is used to produce molten copper and aluminium oxide slag. Exothermic is a chemical term describing a chemical reaction which gives off heat as the reaction takes place.

01-cadwell thermit welding process

The reactions involved may be written as:

3CuO + 2Al à 3 Cu + Al2O3 + Heat

3Cu2O + 2Al à 6 Cu + Al2O3 + Heat

The reactions produce 98 percentage of pure copper. The heat of reaction and the pure copper thus obtained are used in the welding of copper to copper and copper to steel. Some specific applications are in the welding of heavy copper cable connections and in the welding of signal bonds to rails.

The cable welding equipment essentially consists of a combination of crucible mould machined in graphite material or carbon material. This mould is assembled into a “handle and clamp” assembly. The mixture is packed in cartridges and can be stored safely. The resulting welding tool is a compact assembly which is portable.

01-cad welding - Exothermic Welding

History of Cad Welding:

The process was developed in 1938 for welding copper alloy signal bonds to rails. Designed by Dr Charles Cadwell on behalf of ERICO, the process was named CADWELD.

Components of CAD Weld:

1. Mould to fit your conductors.

2. Weld Metal required by your mould including drop metal disk.

3. Handle Clamps or Frame.

4. Flint Ignitor (included with handle clamps and frames).

5. Lugs, sleeves, packing material listed on the page with the mould.

01-cad welding - cadwell welding - thermite welding

Process Steps:

1. The insulation on the cables is removed, the conductors are positioned in the mould cavity and the welding tool is closed and clamped. The cartridge is emptied into the crucible.

2. The bottom of each cartridge is filled with a little inflammable powder for starting. This powder is ignited with a spark gun which sets off the reaction.

3. The molten copper melts the small steel disc preplaced at the bottom of the crucible, and the metal runs into the mould to weld the two conductors together.

4. The resulting cad welded conductor has the tensile strength of soft drawn copper. By deliberately increasing the cross sectional area of the weld, the current carrying capacity of the joint is made equal to or more than of the original conductor.

5. After the welding, a little aluminium oxide slag remains in the crucible of the unit which can be cleaned and the welding tool made ready for the next weld.

01-cadwell weld - cad weld mould


1. Making electrical connections of copper to copper or copper to steel in which no outside source of heat is required.

2. For use in critical ground terminations where a high level of security is required

3. Also ideal for installation in heavy traffic areas

4. Suitable for both paving and hot tar applications

Thermit Process | Thermit Welding Materials | Application Of Thermit Welding

Thermit Welding

Thermit welding is a fusion welding process that makes use of the intense heat produced when a mixture containing iron oxide and powdered aluminium is ignited. The reaction is based on the high affinity of aluminium is ignited.

01-thermit welding materials - thermite nickel - thermite powder

It reduces iron oxide to thermit steel and slag, like this

8Al + 3Fe3O4 – -> 9Fe + 4Al2O3 (Slag) + Heat

The superheated molten metal is poured at the desired place which on solidification forms the welded joint. The process is thus essentially a combination of casting and welding processes. The Thermit mixture consists primarily of finely divided aluminium and iron oxide in the ratio of about 1: 3 by weight. Other metal oxides that can be used in place of iron oxide include oxides of Copper, Nickel, Chromium or Manganese but Iron oxide Thermit is the most commonly used. The mixture is filled in a specially designed refractory crucible and the reaction is started by igniting the mixture with a highly inflammable powder consisting of Barium Peroxide.

When the mixture is ignited, an ignition temperature of 1150 deg Celsius is attained which initiates the main thermit reaction. The reaction is self sustaining and very rapid as it is exothermic. A temperature of the order of 3000 deg Celsius is produced resulting in super heated thermit steel. Slag being very light floats over the thermit steel thereby protecting the metal from atmospheric gases. Apart from the basic ingredients of the thermit mixture other materials may be added to produce a desired thermit melt for any specific application.

01-thermit welding process - thermite rail welding

Types of Thermits used for Ferrous welding applications:

1. Plain thermit

2. Forging thermit

3. Cast iron thermit

4. Wabbler thermit

01-alumium thermite - nickel thermite - thermite composition

Plain Thermit:

It is a mixture of Aluminium and Iron oxide and is the base for all other thermits.

Forging Thermit:

Along with Plain thermit, Nickel, Manganese and Mild Steel are added as a mixture. It is used to weld Steel Components.

Cast Iron Thermit:

Along with Plain Thermit, Ferro-Silicon and Mild Steel are added as a mixture. They are used for Welding Iron Components.

Wabbler Thermit:

Which is specially alloyed to produce a hard, Wear resistant, Machinable Steel used for producing rolls and pinions in rolling mills.

01-thermite process - rail fusion welding

Working Operation:

In making a thermit weld a mould is built around the sections to be welded and is preheated before use. This is done by first cutting the ends of the pieces to be welded to provide a gap with parallel faces. The gap is filled with wax which serves as a pattern for the weld. The ends of the work pieces are enclosed in a suitable flask and moulding sand rammed around the joint taking care to provide openings for the runner, riser and a heating gate.

01-thermite welding kits - Iron oxide thermite

Heat is applied through the melting gate to melt out the wax and to preheat the ends of the pieces to be welded. The heating gate is then plugged with an iron plug or sand core to prevent flow of thermit metal. Thermit reaction is started in the thermit crucible and the resulting superheated steel is let in, from the bottom of the crucible. The slag being lighter floats over the molten metal in the crucible. It flows last and remains at the top of the mould where it solidifies. Clean metal enters the mould around the sections to be welded.

The temperature of the molten steel entering the mould is around 2500 degree Celsius. The heat of the superheated metal fuses and amalgamates the sections together forming joint on solidification. After the joint has solidified the mould is broken open and discarded.

The thermit welding process is useful for welding heavy sections. Though initially used primarily for joining of rails the process can be used for repair work of heavy parts like tracks, spokes of large wheels, broken motor casings, and connecting rods.

01-what is thermit welding - thermite welding procedure


1. The heat necessary for welding is obtained from a chemical reaction and thus no costly power – supply is required. Therefore broken parts (rails etc.) can be welded on the site itself.


1. Thermit welding is applicable only to ferrous metal parts of heavy sections, i.e., mill housing and heavy rails sections.

2. The process is uneconomical If used to weld cheap metals or light parts.